The authors analyse from hemorheological viewpoint the vascular system and the blood, its special circulating medium. The most important anatomical and physiological characteristics are referred to completing them with the presentation of the basic rheophysical parameters and correlations. Factors determining the viscosity of blood are specified and discussed separately. The definition based on literary data according to which the viscosity of blood is greater with lower circulation gradient and lower--in view of the non-Newtonian character of bloodat a higher rate is discussed. The significance of the adaptability of blood cells is emphasized in the determination of the rheological characteristics of blood.